There have been proposed methods of producing insulated wire having a foamed covering layer on a conductive material by foam extrusion molding an insulating material on a conductive material with or without further applying cross-linking treatment or post-foaming treatment to a coating layer formed (Japanese Patent Publication Nos. 37486/1987, 6055/1985).
Conventionally, a polymer composition containing high-density polyethylene, a nucleator such as azodicarbonamide and 4,4'-oxybis(benzenesulfonylhydrazide) and a foaming agent has been known as an insulating material used in the above-mentioned foam extrusion molding.
However, those containing azodicarbonamide as a nucleator pose problem in that they are inferior in insulation properties presumably due to by-production of polar substance as a decomposed residue resultant of decomposition at a foaming temperature, showing high tan .delta. of the foamed insulating layer formed. Also, those containing 4,4'-oxybis(benzenesulfonylhydrazide) need dry treatment of the foamed insulating layer formed subsequent to foam molding in response to generation of water caused by decomposition, rendering them industrially inadequate.
Furthermore, when high-density polyethylene is used as a base resin, use of a conventionally-employed nucleator such as azodicarbonamide or 4,4'-oxybis(benzenesufonylhydrazide) renders production of a foamed insulating layer having uniform, finely-divided foams with high extent of foaming, for example, not less than 70% and excellent insulation properties such as small tan .delta. at high frequency of 0.01-8 GHz difficult.